Determination of Mechanical Properties of Sand-Coated Carbon Fiber Reinforced Polymer (CFRP) Rebar

This experimental study investigates the fundamental mechanical characteristics of the carbon fiber-reinforced polymer (CFRP) bars, including the tensile strength, compressive strength, shear strength, and modulus of elasticity of the CFRP bar. The properties need to be accurately determined to understand the behavior of the concrete structures reinforced with CFRP rebars. The CFRP rebar was coated with sand to enhance the adhesive strength of the concrete. Three diameters of CFRP rebar (D10, D12, and D16) were considered in accordance with ASTM provisions. A coefficient, i.e., the ratio of shear strength to tensile strength, was employed to predict the tensile strength of the CFRP rebar specimens. The test results confirm that the tensile strength of CFRP rebar is dependent on its diameter due to the shear lag effect. A coefficient in the range of 0.17 to 0.2 can be used to predict the tensile strength of CFRP rebar using shear strength.


Introduction
For decades, research into composite materials has explored the feasibility of a replacement material for conventional reinforcement in concrete structures. In Japan, Europe, Canada, and the United States, research for alternative materials for conventional steel rebar has been actively conducted, among them, fiber-reinforced polymer (FRP) composites are employed as an alternative. FRP materials are widely used in the construction industry due to their superior mechanical and physical advantages such as high chemical resistance, high corrosion resistance, lightweight, non-conductivity, etc. FRP rebar is anisotropic material that can be manufactured by either a pultrusion process or braiding technique. The pultrusion process is inexpensive and can rapidly produce a member with a constant cross-section. Composites produced by the pultrusion process are good in structural applications due to their continuous mass production with homogeneous mechanical properties. However, because members generated by the pultrusion process have a smooth surface, an additional step of digging or excavating with a machine and protruding or coating the surface is needed to increase the strength of its bond with the concrete interface. The braiding technique is a modification of the pultrusion process that creates protrusions via a weaving process prior to the hardening stage. Although this braiding method is difficult in practice and the fiber content is less than in the pultrusion process, external protrusions can be created easily. The braiding technique was used to manufacture the carbon fiber-reinforced polymer (CFRP) rebar specimens in this study. In order to improve the adhesion performance of the FRP rebar, surface treatment of the protrusion was preferred. Among FRPs, the CFRP has higher tensile strength and elastic modulus than steel rebar. Based on the previous results, the mechanical performance of the FRP rebar used in construction and civil engineering was verified, and the CFRP rebar was confirmed to be used as a steel rebar substitute [1,2]. Additional studies are needed to

Materials
The FRP rebar has lower adhesion to concrete than steel rebar [15]. The CFRP rebar specimens used in this study were sand-coated to improve the bond strength of the concrete. Figure 1 shows the sand-coated CFRP rebar specimens with diameters of 10 mm, 12 mm, and 16 mm. The manufacturer (SK chemical, Seoungnamsi, Korea) provided the following information regarding the properties: ultimate stress > 2850 MPa (based on ASTM D3039M [16]), modulus of elasticity > 158 GPa, and ultimate strain > 1.8 percent. The ratio of carbon fiber (CF) to total area is about 42%. mers 2022, 14, x FOR PEER REVIEW 3 of 13 D3039M [16]), modulus of elasticity > 158 GPa, and ultimate strain > 1.8 percent. The ratio of carbon fiber (CF) to total area is about 42%.

Tensile Strength Testing of CFRP Rebar Specimens
The tensile strength tests of the CFRP rebar specimens were conducted in accordance with ASTM D7205 [17]. Five CFRP rebars for each of the three diameters, D10, D12, and D16 were prepared for the tensile strength test. The ASTM D7205 standard specifies the tensile strength test method suggested in ACI 440.3R-12 [4]. This test can determine the tensile strength of FRP matrix composite rebar that typically is used as a tensile element in rebar and prestressed post-tension concrete. Table 1 presents the dimensions of the tensile strength test CFRP rebar specimens used in this study. A steel tube (thickness: 2 mm) filled with epoxy at both ends of the specimen was fabricated in accordance with ASTM D7205. Due to the length limitation of testing equipment, the specimen for D16 was designed with a grip length of 660 mm. Figure 2a schematically presents the CFRP D10 rebar specimen fabricated for the tensile strength test. Figure 2a presents the CFRP D10 rebar specimens fabricated for the tensile strength test with a grip length of 550 mm, free length of 400 mm, and total length of 1500 mm. Figure 2b schematically presents the CFRP D12 and D16 rebar specimens fabricated for the tensile strength test with a grip length of 660 mm, free length of 480 mm, and total length of 1800 mm. Figure 2c presents the tensile strength test set-up whereby the load is applied in displacement control mode using a universal testing machine (UTM) with a capacity of 1200 kN at the rate of 3 mm/min.

Tensile Strength Testing of CFRP Rebar Specimens
The tensile strength tests of the CFRP rebar specimens were conducted in accordance with ASTM D7205 [17]. Five CFRP rebars for each of the three diameters, D10, D12, and D16 were prepared for the tensile strength test. The ASTM D7205 standard specifies the tensile strength test method suggested in ACI 440.3R-12 [4]. This test can determine the tensile strength of FRP matrix composite rebar that typically is used as a tensile element in rebar and prestressed post-tension concrete. Table 1 presents the dimensions of the tensile strength test CFRP rebar specimens used in this study. A steel tube (thickness: 2 mm) filled with epoxy at both ends of the specimen was fabricated in accordance with ASTM D7205. Due to the length limitation of testing equipment, the specimen for D16 was designed with a grip length of 660 mm.  Figure 2a presents the CFRP D10 rebar specimens fabricated for the tensile strength test with a grip length of 550 mm, free length of 400 mm, and total length of 1500 mm. Figure 2b schematically presents the CFRP D12 and D16 rebar specimens fabricated for the tensile strength test with a grip length of 660 mm, free length of 480 mm, and total length of 1800 mm. Figure 2c presents the tensile strength test set-up whereby the load is applied in displacement control mode using a universal testing machine (UTM) with a capacity of 1200 kN at the rate of 3 mm/min.

Compressive Strength Testing of CFRP Rebar Specimens
The compressive strength tests were conducted using CFRP rebar specimens with their lengths set to two times the diameter of the specimen in accordance with the compressive strength test method specified in ASTM D695 [18]. Five specimens for each of the three diameters, D10, D12, and D16, were prepared for each compressive strength test. Modulus of elasticity tests were conducted using specimens with their lengths set to four times the diameter of the specimen, as shown in Figure 3. Table 2 provides the dimensions of the compressive strength test CFRP rebar specimens. Note that '2D' and '4D' refer to two times and four times the diameter, respectively.

Compressive Strength Testing of CFRP Rebar Specimens
The compressive strength tests were conducted using CFRP rebar specimens with their lengths set to two times the diameter of the specimen in accordance with the compressive strength test method specified in ASTM D695 [18]. Five specimens for each of the three diameters, D10, D12, and D16, were prepared for each compressive strength test. Modulus of elasticity tests were conducted using specimens with their lengths set to four times the diameter of the specimen, as shown in Figure 3. Table 2 provides the dimensions of the compressive strength test CFRP rebar specimens. Note that '2D' and '4D' refer to two times and four times the diameter, respectively.

Compressive Strength Testing of CFRP Rebar Specimens
The compressive strength tests were conducted using CFRP rebar specimens their lengths set to two times the diameter of the specimen in accordance with the c pressive strength test method specified in ASTM D695 [18]. Five specimens for each o three diameters, D10, D12, and D16, were prepared for each compressive strength Modulus of elasticity tests were conducted using specimens with their lengths set to times the diameter of the specimen, as shown in Figure 3. Table 2 provides the dimens of the compressive strength test CFRP rebar specimens. Note that '2D' and '4D' ref two times and four times the diameter, respectively.     Figure 4a,b show identical compressive strength test set-ups for the specimens with their lengths two times and four times their diameters, respectively. The load was applied in displacement control mode at a rate of 1 mm/min using a 100-kN UTM.

Shear Strength Testing of CFRP Rebar Specimens
The shear strength tests were conducted using five specimens for each of the three diameters, D10, D12, and D16. Figure 5 shows the test specimens that were fabricated with the length of 225 mm in accordance with ASTM D7617 [19]. The set-up required for this test was designed specifically to fit the specimen for each diameter according to the ASTM D7617 standard.

Shear Strength Testing of CFRP Rebar Specimens
The shear strength tests were conducted using five specimens for each of the t diameters, D10, D12, and D16. Figure 5 shows the test specimens that were fabricated with the length of 225 m accordance with ASTM D7617 [19]. The set-up required for this test was designed sp ically to fit the specimen for each diameter according to the ASTM D7617 standard.   in displacement control mode at a rate of 1 mm/min using a 100-kN UTM.

Shear Strength Testing of CFRP Rebar Specimens
The shear strength tests were conducted using five specimens for each of the three diameters, D10, D12, and D16. Figure 5 shows the test specimens that were fabricated with the length of 225 mm in accordance with ASTM D7617 [19]. The set-up required for this test was designed specifically to fit the specimen for each diameter according to the ASTM D7617 standard.   Figure 6a shows the shear jig used for the shear strength test and Figure 6b shows the shear strength test set-up. The load was applied in displacement control mode at the rate of 1 mm/mm using a 100-kN UTM.
in displacement control mode at a rate of 1 mm/min using a 100-kN UTM.

Shear Strength Testing of CFRP Rebar Specimens
The shear strength tests were conducted using five specimens for each of the t diameters, D10, D12, and D16. Figure 5 shows the test specimens that were fabricated with the length of 225 m accordance with ASTM D7617 [19]. The set-up required for this test was designed sp ically to fit the specimen for each diameter according to the ASTM D7617 standard.    Figure 7 shows the tensile strength test specimens and the location and mode of failure. Figure 7a,c show that the CFRP rebar specimens fractured at the center and grip of the specimens, respectively. Figure 7b shows the fracture of the CFRP fiber at the center and that fiber weave is unidirectional. As the fiber weave is generated in one direction, the fibers of the CFRP rebar break sequentially, thus resulting in a brittle fracture. Figure 7d shows a fracture at the grip of the specimen.  Figure 7 shows the tensile strength test specimens and the location and mode of failure. Figure 7a,c show that the CFRP rebar specimens fractured at the center and grip of the specimens, respectively. Figure 7b shows the fracture of the CFRP fiber at the center and that fiber weave is unidirectional. As the fiber weave is generated in one direction, the fibers of the CFRP rebar break sequentially, thus resulting in a brittle fracture. Figure  7d shows a fracture at the grip of the specimen.  Table 3 presents the tensile strength test results for the CFRP rebar specimens in terms of tensile strength value, modulus of elasticity value, and failure mode. Table 3 also provides the average values of the tensile strength and modulus of elasticity.   Table 3 presents the tensile strength test results for the CFRP rebar specimens in terms of tensile strength value, modulus of elasticity value, and failure mode. Table 3 also provides the average values of the tensile strength and modulus of elasticity.  the CFRP D12 rebar specimens were crushed at the point where the load was applied, as shown in the figures. The fibers became separated from each other due to the failure of the resin rather than buckling. Figure 8a-c present the stress-displacement relationship of the D10, D12, and D16 CFRP rebar specimens, respectively. The results clearly indicate that CFRP rebar has no yield point. For the D16 specimens, an unexpected bilinear relationship was observed due to the slippage at the grip area. The average tensile strength value of the five D12 specimens tested is 1784 MPa. This value satisfies the standard tensile strength range of 600 MPa to 3690 MPa for CFRP rebar specified in ACI 440.1R-15 [2]. The average modulus of elasticity value is 158 GPa, which is also within the standard modulus of elasticity range of 120 GPa to 580 GPa for CFRP rebar specified in ACI 440.1R-15. Moreover, these results also satisfy the standard ( Figure 9a,b present photos of compressive strength test D12 specimens with lengths that are two times and four times their diameters, respectively, and their failure modes. All the CFRP D12 rebar specimens were crushed at the point where the load was applied,  Table 4 presents the results of the compressive strength tests for each of the five specimens with each of the three diameters (D10, D12, and D16) and for each of the lengths (two times and four times the diameter, respectively). The compressive strength test results of the CFRP rebar indicate an average compressive strength of 357 MPa in the case of the D12 CFRP rebar. The compressive strength of the CFRP rebar specimen with a   Table 4 presents the results of the compressive strength tests for each of the five specimens with each of the three diameters (D10, D12, and D16) and for each of the lengths (two times and four times the diameter, respectively). The compressive strength test results of the CFRP rebar indicate an average compressive strength of 357 MPa in the case of the D12 CFRP rebar. The compressive strength of the CFRP rebar specimen with a length that is twice the diameter is approximately 7% greater than that of the CFRP rebar specimen with a length that is four times the diameter. In addition, the compressive strength values of the CFRP rebar with lengths that are twice and four times the diameter are 79.9% and 81.2% lower than the tensile strength, respectively. The results of the compressive strength tests of the CFRP rebar indicate that the average compressive strength values of D10 and D16 CFRP rebar are 399 MPa and 360 MPa, respectively. The compressive strength of the CFRP D10 rebar specimen with a length that is twice the diameter is approximately 12% greater than that of the CFRP D10 rebar specimen with the length that is four times the diameter, whereas the compressive strength of the CFRP D16 rebar specimen with the length that is twice the diameter is approximately 9% smaller than that of the CFRP D 16 rebar specimen with the length that is four times the diameter.

Shear Strength Test Results
Shear strength tests of five specimens for each of the three diameters (D10, D12, and D16) were conducted, and Figure 10 shows a photo of the typical shear failure observed for the D12 specimens.

Shear Strength Test Results
Shear strength tests of five specimens for each of the three diameters (D10, D12, and D16) were conducted, and Figure 10 shows a photo of the typical shear failure observed for the D12 specimens.       Figure 11 shows the stress-displacement relationships of five shear strength test specimens for each of the three dimensions. Figure 11a shows that the stress increased up to 371 MPa for the CFRP rebar D10. Figure 11b shows that the stress of the CFRP rebar D12 test specimens continuously increased until failure. The CFRP rebar D16 had smaller shear strength than CFRP rebars D10 and D12, as shown in Figure 11c. According to the results of shear strength tests of FRP in previous studies [20], CFRP rebar maintains constant stress before failure and exhibits failure in terms of horizontal and vertical cracks. However, by contrast, the shear strength test results obtained in this study indicate that CFRP rebar shows a tendency to fracture immediately without resistance to a constant load. The reason for this outcome appears to be due to the FRP weaving method. The CFRP specimens used in this study showed significant resistance to loading in the longitudinal direction because they were fabricated in one direction and thus were vulnerable to shear. Therefore, specimens should be fabricated based on three dimensions instead of using single-directional weaving methods to improve the shear performance of CFRP rebar. D12 test specimens continuously increased until failure. The CFRP rebar D16 had smaller shear strength than CFRP rebars D10 and D12, as shown in Figure 11c. According to the results of shear strength tests of FRP in previous studies [20], CFRP rebar maintains constant stress before failure and exhibits failure in terms of horizontal and vertical cracks. However, by contrast, the shear strength test results obtained in this study indicate that CFRP rebar shows a tendency to fracture immediately without resistance to a constant load. The reason for this outcome appears to be due to the FRP weaving method. The CFRP specimens used in this study showed significant resistance to loading in the longitudinal direction because they were fabricated in one direction and thus were vulnerable to shear. Therefore, specimens should be fabricated based on three dimensions instead of using single-directional weaving methods to improve the shear performance of CFRP rebar.

Effect of Size of CFRP Rebar
With an increase in the diameter of CFRP rebar, the tensile strength tends to decrease

Effect of Size of CFRP Rebar
With an increase in the diameter of CFRP rebar, the tensile strength tends to decrease due to an uneven tensile stress distribution throughout the cross-section. This result is matched well with the experimental results in the literature [21]. Furthermore, unlike steel rebar, CFRP rebar has an orientation due to its fiber inclusion, and both the strength and stiffness of CFRP rebar vary according to the fiber content and resin used. Therefore, the tensile strength tests should be conducted using many specimens in order to ensure the reliability of the material's mechanical performance. Similarly, the shear strength is significantly affected by the size of CFRP rebar due to the fiber matrix and resin. The shear strength is reduced as an increase in the CFRP rebar size as shown in Figure 12. Regardless of the CFRP rebar size, the tensile modulus of elasticity of CFRP rebars was higher than their compressive modulus of elasticity. The results are matched well with the findings in the literature [10]. strength is reduced as an increase in the CFRP rebar size as shown in Figure 12. Regardless of the CFRP rebar size, the tensile modulus of elasticity of CFRP rebars was higher than their compressive modulus of elasticity. The results are matched well with the findings in the literature [10]. Jung et al. [22] reported the modulus of elasticity of hybrid rebar to be approximately 100 GPa. The CFRP D12 rebar investigated in this study showed a modulus of elasticity value that is 1.58 times higher than that of the hybrid rebar but lower than that of steel rebar (200 GPa). Plevkov et al. [12] found that the modulus of elasticity value of general CFRP rebar is 144 GPa, which is 0.91 times lower than that of the CFRP D12 rebar used in this study. Therefore, the CFRP rebar produced in the future as steel rebar replacement should have enhanced strength, which can be accomplished by conducting additional research studies using different fiber arrangements/orientations and different resin contents.

Relationship between Shear Strength and Tensile Strength
As addressed in the literature [10], the tensile test for the CFRP rebar needs high attention to avoid premature failure in tension. As increasing CFRP rebar size, the test setup required a large free length, enough steel pipe anchor, and a high-capacity loading machine. Simply, a correlation between tensile strength and shear strength can be employed to predict the tensile strength of CFRP rebar according to the rebar's diameter. Equation (1) can be used to calculate the ratio of shear strength to tensile strength using the results of shear and tensile strength tests of CFRP rebar with a diameter up to 16 mm [21].
where kd is the coefficient; fs is the shear strength; ft is the tensile strength. Figure 13 shows the computed coefficients (kd). The literature shows that these coefficients (kd) range from 0.163 to 0.207 for basalt FRP rebar and basalt/CFRP hybrid rebar depending on the diameter of the rebar [21]. The values obtained in this study match well Jung et al. [22] reported the modulus of elasticity of hybrid rebar to be approximately 100 GPa. The CFRP D12 rebar investigated in this study showed a modulus of elasticity value that is 1.58 times higher than that of the hybrid rebar but lower than that of steel rebar (200 GPa). Plevkov et al. [12] found that the modulus of elasticity value of general CFRP rebar is 144 GPa, which is 0.91 times lower than that of the CFRP D12 rebar used in this study. Therefore, the CFRP rebar produced in the future as steel rebar replacement should have enhanced strength, which can be accomplished by conducting additional research studies using different fiber arrangements/orientations and different resin contents.

Relationship between Shear Strength and Tensile Strength
As addressed in the literature [10], the tensile test for the CFRP rebar needs high attention to avoid premature failure in tension. As increasing CFRP rebar size, the test setup required a large free length, enough steel pipe anchor, and a high-capacity loading machine. Simply, a correlation between tensile strength and shear strength can be employed to predict the tensile strength of CFRP rebar according to the rebar's diameter. Equation (1) can be used to calculate the ratio of shear strength to tensile strength using the results of shear and tensile strength tests of CFRP rebar with a diameter up to 16 mm [21].
where k d is the coefficient; f s is the shear strength; f t is the tensile strength. Figure 13 shows the computed coefficients (k d ). The literature shows that these coefficients (k d ) range from 0.163 to 0.207 for basalt FRP rebar and basalt/CFRP hybrid rebar depending on the diameter of the rebar [21]. The values obtained in this study match well with the results from the literature. Therefore, the calculated coefficients (k d ) can be used to predict the tensile strength of CFRP D10 to D16 rebar.

Conclusions
Tensile, compressive, and shear-strength tests were conducted in this study in accordance with the test methods specified in ASTM and other international standards to evaluate the mechanical performance of CFRP rebar. As a result of the tests and quantification of the mechanical performance of the CFRP rebar specimens, the following conclusions can be drawn.
The results of the tensile strength tests of CFRP rebar conducted in accordance with ASTM D7205 indicate that the average tensile strength value is 1784 MPa and the average modulus of elasticity value is 158 GPa. This value satisfies the standard modulus of elasticity values specified in ACI 440.1R-15 (in the range of 120 GPa to 580 GPa) and the minimum modulus of elasticity value of 140 GPa specified in guidelines for the structural design of FRP rebar.
For the compressive strength and modulus of elasticity tests, the test results indicate that the average compressive strength value is 357 MPa and the average modulus of elasticity value is 30 GPa. The tensile modulus of elasticity of CFRP rebars was higher than their compressive modulus of elasticity.
As the results of the shear strength test, all specimens are continuously increased until failure. In short, the shear strength and tensile strength of CFRP rebar are affected by the CFRP rebar size. In addition, as a result of investigating the relationship between the shear strength and tensile strength of CFRP, it was possible to predict the coefficient according to the diameter of the CFRP rebar.
Based on the mechanical performance testing, the sand-coated CFRP rebar was determined in this study. The following study will use this information for further structural tests and analyses. To achieve the target tensile strength value of 2100 MPa in future research, the resistance to tension should be increased throughout the polymer section by changing the fiber arrangement/orientation of the CFRP rebar to improve its brittle-resistant properties. In addition, to prevent grip failure and slippage, an adequate bond system along with ASTM specification is needed for future tests.

Conclusions
Tensile, compressive, and shear-strength tests were conducted in this study in accordance with the test methods specified in ASTM and other international standards to evaluate the mechanical performance of CFRP rebar. As a result of the tests and quantification of the mechanical performance of the CFRP rebar specimens, the following conclusions can be drawn.
The results of the tensile strength tests of CFRP rebar conducted in accordance with ASTM D7205 indicate that the average tensile strength value is 1784 MPa and the average modulus of elasticity value is 158 GPa. This value satisfies the standard modulus of elasticity values specified in ACI 440.1R-15 (in the range of 120 GPa to 580 GPa) and the minimum modulus of elasticity value of 140 GPa specified in guidelines for the structural design of FRP rebar.
For the compressive strength and modulus of elasticity tests, the test results indicate that the average compressive strength value is 357 MPa and the average modulus of elasticity value is 30 GPa. The tensile modulus of elasticity of CFRP rebars was higher than their compressive modulus of elasticity.
As the results of the shear strength test, all specimens are continuously increased until failure. In short, the shear strength and tensile strength of CFRP rebar are affected by the CFRP rebar size. In addition, as a result of investigating the relationship between the shear strength and tensile strength of CFRP, it was possible to predict the coefficient according to the diameter of the CFRP rebar.
Based on the mechanical performance testing, the sand-coated CFRP rebar was determined in this study. The following study will use this information for further structural tests and analyses. To achieve the target tensile strength value of 2100 MPa in future research, the resistance to tension should be increased throughout the polymer section by changing the fiber arrangement/orientation of the CFRP rebar to improve its brittle-resistant properties. In addition, to prevent grip failure and slippage, an adequate bond system along with ASTM specification is needed for future tests.